In a digital coherent transmission technology, which performs coherent detection on a phase-modulated polarization-division multiplexed optical signal and restores a signal using digital signal processing, the transmission capacity per wavelength channel and the spectral efficiency are increasing dramatically (e.g., see Non-Patent Documents 1 to 3).
Digital signal processing makes demultiplexing of a polarization-division multiplexed signal, frequency offset compensation of local oscillation light, chromatic dispersion compensation, polarization mode dispersion compensation, and band-limited distortion compensation possible. Since the polarization state of signal light, polarization mode dispersion of a transmission path, the amount of a frequency offset of local oscillation light, and so on vary over time, a digital signal processing circuit generally includes an adaptive equalization filter. Representative control algorithms of adaptive equalization filters include the constant modulus algorithm (CMA) and the decision-directed least mean squared (DD-LMS) algorithm. These control algorithms calculate a transfer function for compensating for waveform distortion due to a transmission path in real time using known information on signal waveforms, thereby realizing an adaptive operation of an equalization filter (e.g., see Non-Patent Document 4).
As another method for compensating for signal distortion, there is a method using maximum likelihood sequence estimation (MLSE). The MLSE is a technique of applying a transmission-path model to a plurality of signal sequence candidates to generate a plurality of reference signals and evaluating the differences between an input signal and the reference signals, thereby estimating the most likely transmission time sequence.